1. Field of the Invention
The present invention relates to a semiconductor element formed by using a semiconductor film having a crystal structure (also referred to as crystalline semiconductor film) and its fabrication method and a semiconductor device provided with a circuit obtained by integrating the semiconductor element and its fabrication method. The present invention relates to a field effect transistor in which a channel-forming region is formed by a crystalline semiconductor film formed on an insulating surface, particularly to a thin-film transistor.
2. Related Art
An art is developed which forms a semiconductor element such as a thin film transistor using a crystalline semiconductor film formed on an insulating substrate such as glass. An art for forming a TFT on a glass substrate is greatly advanced and application of the art to an active-matrix semiconductor display which is one of semiconductor devices is progressed.
Particularly, an art for crystallizing an amorphous semiconductor film by irradiating the film with a laser beam is applied to an art for fabricating a thin-film transistor (TFT). A glass substrate is regarded to be prospective as a substrate used for a semiconductor device from the viewpoint of cost compared to a single-crystal-silicon substrate. A glass substrate is inferior in heat resistance and is easily deformed. Therefore, when forming a polysilicon TFT on a glass substrate, it is very effective to use a laser annealing method for crystallization of a semiconductor film in order to avoid thermal deformation of a glass substrate.
Because a TFT using a polycrystalline semiconductor film (polysilicon TFT) has a field-effect mobility (also referred to as mobility) higher than that of the conventional TFT using an amorphous semiconductor film, it realizes high-speed operations. Therefore, it is possible to perform pixel control which has been performed so far by a driving circuit set outside a substrate by a driving circuit formed on the same substrate as pixels. A thin-film transistor fabricated by a crystalline semiconductor film is applied to a planar display (flat panel display) represented by a liquid-crystal display or an EL (electroluminescence) display.
The laser annealing method has features that treatment time can be greatly decreased compared to the annealing method which uses radiation heating or conduction heating and a substrate is hardly thermally damaged because it is selectively or locally heated.
The above laser annealing method represents an art for recrystallizing a damaged layer formed on a semiconductor substrate or semiconductor film or an art for crystallizing a semiconductor film formed on a substrate. Moreover, the laser annealing method includes an art to be applied to flattening or surface reforming of a semiconductor substrate and semiconductor film. A laser oscillator to be applied is a gas laser oscillator represented by an excimer laser or a solid laser oscillator represented by a YAG laser, which is known as an oscillator for crystallizing the surface layer of a semiconductor by irradiating the surface layer with a laser beam and thereby heating it for a very short time such as several tens of nanoseconds to several tens of microseconds.
Lasers are roughly divided into two types such as pulse oscillation and continuous oscillation in accordance with the oscillation method of a laser. Because an output of the pulse oscillation has a comparatively high output energy, it is possible to improve the mass productivity by setting the size of a laser beam to several square centimeters. Particularly, by forming the shape of a laser beam like a line having a length of 10 cm or more by an optical system, it is possible to efficiently apply a laser beam and further improve the mass productivity. Therefore, it had been a main stream to use a pulse-oscillation laser for crystallization of a semiconductor film.
However, it is recently found that the grain diameter of crystal formed in a semiconductor film increases by using a continuous-oscillation laser compared to the case of using a pulse-oscillation laser for crystallization of the semiconductor film. When the crystal grain diameter in a semiconductor film increases, the mobility of a TFT formed by the semiconductor film rises. Therefore, the continuous-oscillation laser is suddenly started to get into the spotlight.
In a semiconductor fabrication process, a gas laser represented by an excimer laser or a solid laser represented by a YAG laser is normally used as the light source of a laser beam. The following Patent Document 1 discloses an art for crystallizing an amorphous semiconductor film by irradiating it with a laser beam, that is, an art for polycrystallizing an amorphous semiconductor film through high-speed scanning without completely melting the film by setting the scanning rate of a laser beam to a beam-spot diameter×5,000/sec or higher. The following Patent Document 2 discloses an art for substantially forming a single-crystal region by irradiating a semiconductor film formed like an island with an extended laser beam. Moreover, a method for applying a laser beam by forming the laser beam into a line by an optical system is known like the case of the laser treatment apparatus disclosed in Patent Document 3.
Patent Document 1
Official gazette of Japanese Patent Laid-Open No. 104117/1987 (p. 92)
Patent Document 2
Specification of U.S. Pat. No. 4,330,363 (FIG. 4)
Patent Document 3
                Official gazette of Japanese Patent Laid-Open No. 195357/1996 (pp. 3-4, FIGS. 1-5) Moreover, the following Patent Document 4 discloses an art for fabricating a transistor by using a solid laser oscillator such as an Nd:YVO4 laser and thereby irradiating an amorphous semiconductor film with a laser beam which is the second harmonic of the laser oscillator to form a crystalline semiconductor film having a crystal grain diameter larger than the conventional one.Patent Document 4        
Official gazette of Japanese Patent Laid-Open No. 144027/2001.
A crystalline semiconductor film formed by the laser annealing method is generally realized when a plurality of crystal grains are collected. Because positions and sizes of the crystal grains are random, it is difficult to form a crystalline semiconductor film by specifying positions and sizes of crystal grains. Therefore, an interface (grain boundary) between crystal grains may be present in an active layer formed by patterning the crystalline semiconductor film like an island.
A grain boundary is one of lattice defects, which is classified into a plane defect and also referred to as a crystalline interface. Plane defects include not only a grain boundary but also a twin plane and a stacking fault. In the case of this specification, however, electrically active plane defects respectively having a dangling bond, that is, a grain boundary and a stacking fault are generally known as a grain boundary.
Recombination centers and trapping centers due to an amorphous structure or crystal defect are innumerably present in a grain boundary differently from the inside of a crystalline interface. When a carrier such as an electron or positive hole is trapped by the trapping center, the potential of the grain boundary rises and works as a barrier for the carrier. Therefore, it is known that the current carrying characteristic of the carrier is deteriorated, that is, the mobility of the electron or positive hole is deteriorated. Therefore, when a grain boundary is present in the active layer of a TFT, particularly in a channel-forming region, it seriously affects TFT characteristics because the mobility of a TFT extremely deteriorates, on-current decreases, or off-current increases because current flows through a grain boundary. Moreover, characteristics of a plurality of TFTs fabricated by assuming that the same characteristics are obtained are fluctuated due to presence or absence of a grain boundary in an active layer.
When irradiating a semiconductor film with a laser beam, positions and sizes of obtained crystal grains become random because of the following reasons. That is, it takes some time until a solid-phase nucleus is generated in a liquid semiconductor film completely melted due to irradiation with a laser beam. Moreover, countless crystal nucleuses are produced in the completely-melted region with elapse of time and crystals are grown from the crystal nucleuses. Because positions of the crystal nucleuses are random, the crystal nucleuses are unevenly distributed. Moreover, because crystal growth ends when crystal grains collide with each other, positions and sizes of the crystal grains become random.
Thus, by irradiating an amorphous semiconductor film formed on a flat surface with a laser beam and thereby crystallizing it, crystal becomes polycrystal, a defect such as a crystalline interface is optionally formed, and thereby it is difficult to obtain the crystal in which orientations are arranged. Particularly, it is difficult to obtain the crystal in which orientations are arranged on a glass substrate by a laser crystallization art.
Moreover, it is impossible to form a semiconductor film in which no strain or crystal defect is present because of volume shrinkage of a semiconductor film caused by crystallization or mismatch of thermal stress or lattice with a ground. Therefore, it is impossible to obtain the same quality as a MOS transistor formed on a single-crystal substrate from a crystalline semiconductor film formed on an insulating surface and crystallized or recrystallized.
In the case of the above planar display, a transistor is built in by forming a semiconductor film on an inexpensive glass substrate. However, it is almost impossible to arrange transistors so as to avoid a crystalline interface to be optionally formed. That is, it is impossible to strictly control the crystallinity of the channel-forming region of a transistor and exclude an unintentionally-included crystalline interface or crystal defect. As a result, electrical characteristics of a transistor are deteriorated and moreover, individual element characteristic is fluctuated.
Particularly, when forming a crystalline semiconductor film on a non-alkali glass substrate industrially frequently used by a laser beam, there is a problem that the focus of the laser beam is fluctuated due to the swell of the non-alkali glass substrate and resultantly crystallinity is fluctuated. Moreover, in the case of the non-alkali glass substrate, it is necessary to use a protective film such as an insulating film as a ground film. Therefore, it is almost impossible to form a crystalline semiconductor film excluding a crystalline interface or crystal defect on the ground film at a large grain diameter.
It is ideal to form a channel-forming region which greatly affects TFT characteristics by single crystal grains by excluding the influence of a grain boundary. However, it is almost impossible to form an amorphous silicon film in which no grain boundary is present by the laser annealing method. Therefore, a TFT using a crystal silicon film as an active layer crystallized by the laser annealing method having the same characteristic as a MOS transistor formed on a single-crystal silicon substrate has not been obtained yet.
The present invention is made to solve the above problems and its object is to provide an inexpensive semiconductor device constituted by a semiconductor element or a group of semiconductor elements capable of performing high-speed operations, having a high current-driving capacity, and having a small fluctuation between a plurality of elements and obtained by forming a crystalline semiconductor film having no crystalline interface in at least a channel-forming region on an insulating surface, particularly an insulating surface using an inexpensive glass substrate as a support substrate.
It is another object of the present invention to provide fabrication method of a semiconductor display device using a laser crystallization method capable of preventing the mobility of a TFT from being extremely deteriorated due to a grain boundary, on-current from decreasing, or off-current from increasing and a semiconductor display fabricated by using the fabrication method.
Moreover, when analyzing at least the above crystalline semiconductor film having no crystalline interface is present in a channel-forming region by the EBSP (Electron Backscatter diffraction Pattern) method, it is a problem to decide a crystal plane which most frequently appears on the main surface as a plane {110} (the crystal plane is referred to as predominant crystal plane). The EBSP method will be described later.
A crystal plane is expressed by putting it in parentheses so that the Miller index is shown by (110) and an equivalent crystal plane such as (101) or (011) is expressed by braces as shown by {110}. Moreover, a crystal orientation (crystal axis) is expressed by putting it in angle brackets as shown by [110] and an equivalent crystal orientation such as [101] or [011] is expressed as shown by <110>.